Micro-crystalline bonding coats for metal



MICRO-CRYSTALLINE BONDING COATS FOR METAL Leo P. Curtin, Cranbury, NJ.

No Drawing. Application October 30, 1957 Serial No. 693,259

8 Claims. (Cl. 148-616) This application is a continuation-in-part of my copending applications, Serial Number 499,510, filed April 5, 1955, now abandoned, and Serial Number 537,900, filed September 30, 1955, now Patent No. 2,846,432, issued Aug 5, 1958.

This invention relates to oxidation-inhibiting and bonding coats on aluminum, zinc and ferrous metal surfaces and it comprises wetting a metal surface free from waterbreaks with a solution containing a dichromate of a divalent metal at least as high as iron in the electromotive series, the solution also containing a reducing agent which is comparatively inert at atmospheric temperatures but which actively reduces chromate to the trivalent form at temperatures above 100 C., the solution also containing at least one phosphate of the class of dihydrogen phosphates of divalent metals at least as high as iron in the electromotive series and free phosphoric acid, the bonding coat being formed by destroying substantially completely the oxidants and reducing agents in the film of chromate solution adhering to the metal surface by heating said film to a temperature of at least 100 C., the residue remaining on the metal surface constituting a highly adherent, oxidation-resisting, chemically inert, micro-crystalline bonding coat; all as is more fully hereinafter set forth and as claimed.

Rust-inhibiting and bonding coats formed on metal by means of an aqueous solution are of three types, principally: (a) phosphate coats, invented by Coslett, U.S.P. 870,937; (b) oxalate coats, invented by Gravell, U.S.P. 1,315,017; (c) chromate treatments, at least as old as Lang, U.S.P. 1,082,161. At present, the most important use of chromate in metal coating is as a 0.10 percent chromic acid solution,

applied over an iron oxide surface previously developed on the metal by means of oxygen at high temperature or by means of a nitric acid or other oxidizing solution, U.S.P. 2,210,850, Curtin, describes the first of such proc esses.

At the present time, well over 90 percent of all bondmg coats on metal are phosphate coats and these are, in applied in the following manner. The

applied as a rinse over a previous ly formed phosphate coating. Also, chromate has been at atmospheric temperatures.

sheet steel or other metal is first thoroughly cleanedinan alkaline solution, moved by a water rinse. The cleaned metal isnext im mersed in a hot phosphate coating bath and kept there until a coating of the desired thickness or weight is formed. Equivalent results are obtained by subjecting the metal to a continuous spray of hot phosphating solution until the desired coating is obtained. A water rinse follows to remove all coating solution and non-adherent sludge. The phoshpate coating may then be given a rinse with a 0.10 percent solution of chromic acid, although this is not an essential part of the process. Drying may be done at any convenient temperature, as by air-drying, since there is nothing required beyond removal of the moisture from the phosphated surface.

the adherent solution then being reof the solution would be at once predetermined rate and the nitn'te content of the bath also requires frequent checking.

The process of the present invention is carried out in three steps: (1) freeing the surface of the metal from water-breaks, (2) wetting the surface with a chromate phosphate solution and (3) destroying oxidants and reducing agents in the film of chromate-phosphate solution adhering to the metal by heating to above 100 C., the residue remaining on the metal surface constituting the desired bonding coat.

In regard to step (1), there are a number of means for reaching this objective. In some cases, metal may be freed from water-breaks by heating substantially above 100 C. Mechanical working of metal, as in the manufacture of steel strip or sheet by the cold-rolling process accomplishes the same result. By far the greatest amount of metal prepared for coating is treated with hot, alkaline solutions which remove dirt and grease and, if properly applied, leave a surface free from water-breaks. A solution containing about '2 percent of sodium metasilicate at 60 to C. has been found to be unusually effective in connection with the present invention. This art is very well understood and is currently practiced on a very large scale. It will be evident that elimination of water-breaks in some fashion is an present process as such areas would be wetted imperfectly or not at all by the chromate solution with consequent impairment of the coating. 1

In regard to step (2), various formulae are given for the chromate solution in the examples which follow.

contains phosphate in the form The chromate solution of a divalent metal dihydrogen phosphate, or phosphoric acid, or both. The phosphate present forms part of the coating, usually as insoluble divalent metal phosphates although, in some cases, a little chromic phosphate, CrPO is formed, this substance being a desirable component of the coating.

The solution also contains a reducing agent for chromate which actively reduces hexavalent chromium to the trivalent form above C. while being relatively inert This last property is essential, otherwise, the chromate would be reduced immediately to the trivalent form, before being applied to metal. coating properties, when applied to metal, the usefulness destroyed it the reducing agent reacted with chromate at the ordinary temperature. It is essential, therefore, that the chromate and reducing agent be capable of existing together in the same solution at ordinary temperatures, for a reasonable time,

perhaps one to several weeks, with no particular immightjbe used in the present invention. One of the best is hypophosphorous acid which works equally well in the form of its divalent metal salts or as ammonium hypophosphite. The reaction is as follows: i

1:. The' zinc chromite formed is a coating component;

Patented Aug. 25, 1959 essential part of the Since a solution of trivalent chromium has no.

reducing substances which and dust-free with no non-adherent material.

The calcium dihydrogen phosphate reacts further with the under-lying metal or with substances present in the solution to form more basic phosphates which are also coating components.

Many organic compounds are desirable reducing agents for chromate, especially those which oxidize to carbon dioxide and water. Such substances as fuma-ric, maleic, citric, lactic, oxalic and hydroxyacetic acids are in this class. Formic acid is less desirable since it tends to vaporize before reducing chromate.

Polyhydroxy alcohols, as glycerol, propylene glycol, trimethylene glycol and ethylene glycol are most desirable, the last-named being particularly useful. The reaction using ethylene glycol is as follows:

The calcium chromite formed is a coating component. Similar reactions take place with many other organic reducing substances. 7

Fatty alcohols containing from about 12 to about 18. carbon atoms, for example, n-dodecyl alcohol, when made soluble by mono-esterification with a suitable polybasic acid, are able to function not only as a reducing agent but, in the form of the divalent metal salts of the resulting fatty acids, as zinc laurate, are capable of formingpart of the coating and increasing its waterrepellant properties. Compounds of the type of n-dodecyl phosphoric acid mono-ester are especially suitable since both the fatty acid and the free phosphoric acid resulting from oxidation produce coating components.

Certain other derivatives of the fatty alcohols function in similar fashion. For example, such substances as the myristamide of bis-diethanolamine and the polyethoxyethanol derivatives of stearic acid oxidize in the chromate solution with production of the free fatty acid which enters the coating in the manner described. Polyethoxy derivatives of fatty alcohols and amines. function in analogous manner.

Because of the complexity of the various chemical reactions involved, which include a slight attack on the metal being coated by hydrogen-ion from the phosphates present, optimum results are often obtained by using the reactants in somewhat different proportions than shown by Reactions A and B.

In many cases, a small amount of free acid is needed to stabilize the solution against hydrolysis with precipitation of such substances as barium chromate and zinc monohydrogen phosphate. A small concentration of free phosphoric or chromic acid checks such tendencies. When the reducing agent present is a fairly strong acid, as maleic, such acid also inhibits hydrolysis. When the acid is destroyed in coating formation, the need for stabilizing the solution no longer exists.

In regard to step (3), the heating of the film of chromate solution adhering to the metal surface to destroy oxidants and reducing agents may be accomplished in many different Ways. Perhaps the most convenient is to utilize a current of air or other gas preheated to 1-50, 200 or 300 C. Such a treatment produces the coating very quickly, the elimination of moisture being accomplished incidentally.

A competent chemist would probably expect that such a treatment of the film of chromate solution would leave a residue of powdery, non-adherent material which could be brushed olf the metal surface, in fact, he would have no basis for any other expectation. Most surprisingly, the coating which is actually formed is perfectly continuous, thin, highly adherent, hard, smooth, lustrous The coating has 'amicro-crystallirie to amorphous structure, also. unpredictable, and it is this feature which permits of re,-

peated flexing through 360 amt deep-drawing up to the point where the. metal itself tears.

The rnicro c'rystalline structurejis believed to result from the great rapidity of the coating formation, also, to the fact that the chromites and phosphates are forced to crystallize in the presence of each other.

It might also be expected that the coating, if formed, would have properties which are the sum of the phosphate and chromite components. Such, however, is not the case; the chromite-phosphate coatings of the present invention are definitely superior to ordinary phosphate coatings or to simple chromite or chromic hydroxide coatings in the matter of resistance to oxidation and greatly superior to such coatings with respect to deepdrawing and similar fabricating operations. Ordinary phosphate coatings are particularly inferior when subjected to drawing and other deforming operations. The phosphate coatings are relatively coarsely crystalline and, in deep-drawing, the hard brittle, crystals tear loose and scratch both the work and the die. 7

Coatings of the present invention take vinyl lacquer coatings perfectly, in fact, they act as stabilizers of the vinyl resins against thermal decomposition. The coatings containing calcium as the only divalent ion are particularly noteworthy in this respect. This important property, also unforeseen, is of great value in the baking of the vinyl resins on iron and other metals which catalytically decompose them.

The following examples show various formulations of the chromate-phosphate solution which is used in step- (2). Usually, the chromate is made into a 25 percent solution and a second solution is made of the reducing agent and the phosphate components. These are combined in the proper proportions at the point of use. The substances other than water are on an anhydrous basis, and in parts by weight.

Example 1: Parts Zinc dichromate 2.8 Magnesium dihydrogen phosphate 0.4 Phosphoric acid 0.1 Hydroxyacetic acid 0.8 Water 95.9:

This is a general purpose formula, suitable for zinc, aluminum and steel;

Example 2: Parts Zinc dichromate 11.2 Zinc dihydrogen phosphate 1'.2 Phosphoric acid 0.5 Ethylene glycol z 2.3 Water 84.8

This concentrated formula is adapted to the spray treatment of preheated metal, especially sheet steel, where excess water is undesirable.

Example 3: Parts Magnesium dichromate I 2.4 Magnesium dihydrogen phosphate 0.5 Ethylene glycol 0.6 Water 96.5

The formula of Example 3 is especially useful for zinc and aluminum.

51 Example is particularly desirable for the coating of sheet steel to be made into food containers.

Example 7 shows a general purpose formula giving coatings especially high in corrosion resistance.

Example 8: Parts Calcium dichromate 2.4 Chromic acid 0.2 Calcium dihydrogen phosphate 0.4 Ethylene glycol 0.6 Water 96.4

The formula of Example 8 is well adapted for the coating of metal to be lacquered and then stamped or drawn.

Example 9: Parts Calcium dichromate q. 13.0 Calcium dihydrogen phosphate 2.5 Phosphoric acid 1.0 Ethylene glycol 3.0 Water 80.5

Example 9 shows a concentrated formula for use on preheated steel sheet for food containers.

Example 10: Parts Barium dichromate 3.5 Chromic acid 0.2 Phosphoric acid 0.3 Maleic acid 0.3 Water 95.7

Example 10 is a general purpose formula for zinc,

aluminum and iron.

Parts Example 11 shows a formula giving coatings in which the water repellency is increased by inclusion of calcium laurate derived from the dodecylester. This formula also iswell adapted for coating aluminum surfaces and for sheet steel for food containers.

The coating formed on metal surfaces by means of the present invention usually weighs from about 1 ounce to about 4 ouncesper one thousand square feet of coated surface. If much below 1 ounce, the coatingmight show discontinuous areas. 1 /2 ounces to 3 ounces is the general purpose range, especially when the metal is to be later subjected to deep-drawing. There does not seem to be much advantage, in most cases, in producing coatings heavier than 4 ounces per 1,000 square feet. The weight of the coating can be precisely controlled by means of the concentration of the chromate-phosphate solution, or by rollers, or by a regulated mist-like spray.

As a first specific embodiment of my invention, in the coating of sheet steel formed into automobile fenders, in step (1), the fender, suspended from a continuously moving conveyor, is subjected to a spray of a solution containing 2 percent sodium eta'silicate and 0.2 percent of dodecylbenzene sodium sulfonate at C. until the metal surface is free from water-breaks, the time varying with the amount of dirt and grease on the surface. Before passing to step (2), the silicate solution adhering to the metal is removed by a water spray.

In step (2), the clean fender is sprayed for about five seconds with a chromate solution at atmospheric temperature formulated as in Example 8, then allowed to drain for 10 seconds. The drainage goes back to the chromate reservoir and is re-used.

The fender, with its adhering film of chromate-phosphate solution, then passes to step (3) where, in a suitable chamber, it encounters a stream of air pre-heated to 200 C. Coating formation takes place quickly and the dry, coated fender, after cooling, may be lacquered immediately.

In a second specific embodiment, corrugated, galvanized structural sheet is coated at the point of manufacture following the procedure of the first embodiment, above, except, in step (1), the aqueous solution is a 3 percent solution of trisodium phosphate at 70. C. and, in step (2), the formula of Example 1 is used.

In a third embodiment, sheets of freshly rolled aluminum are coated according to the procedure of the first embodiment except that, in step (1), the solution contains 0.5 percent sodium oleate, 1.0 percent free ammonia and 2.0 percent of diammonium phosphate and, in step (2), the chromate-phosphate solution is that of Example 11.

Embodiments 1, 2 and 3, above, may be carried out satisfactorily by dipping instead of spraying and the surplus chromate solution in step moved by an air blast.

In a fourth specific embodiment, in the coating of fair- 1y clean, continuous strip sheet steel, in step (1), the moving strip is sprayed on both sides with an aqueous solution of 1.5 percent sodiummetasilicate and 0.1 percent dodecyl benzene sodium sulfonate at 70 C. for 4 seconds per linear foot, the excess silicate solution being largely removed by corrugated rollers and the remainder by a jet of water.

The strip moves to step (2) where it is wetted, both sides, with a fine spray of chromate-phosphate solution at 20 to 40 C. for about 2 seconds per linear foot, the solution being formulated as in Example 5. The strip then passes under and over corrugated rollers which regulate the amount of solution left on the surface. The

rolled, continuous strip steel is given the bonding coat at the time of rolling, step (1) is taken care of by the coldrolling operation. The rapidly moving steel strip, still Very hot from the rolling, is subjected, both sides, to a mist-like chromate-phosphate spray, with no run-ofi, using the formula of Example 9. Both the application of the chromate solution, step 2), and the formation of the coating, step (3), take place in less than 2seconds per linear foot of strip, the residual heat of the stripbeing utilized to form the coating. The resulting coated strip may be coiled up, or cut into lengths, or lacquered in the flat.

Many other modifications of the operating details. of the present coating process will be evident. Although there are an enormous number of items of zinc, aluminum and ferrous metal which require bonding coats, the

process ofjthe present invention can easily be adapted to coat all of them. i The present process for applying micro-crystalline,

(2) may be quickly re- 7 chromite-phosphate coats to metal differs from known art in the following respects:

The micro-crystalline chromite-phosphate bonding coats of the present invention are unequalled by any known bonding coat in oxidation resistance and capacity for withstanding flexing, deep-drawing and other deforming operations. They contain substantially no chromate or other soluble salts. The coatings are hydrophobic but are Wetted with great ease by organic solvents, thus making a perfect bond between bonding coat and lacquer. They are compatible with, and stabilizers for, the highly important vinyl resin lacquers.

-What I claim is:

1. Process for producing an adherent, micro-crystalline chromite-phosphate bonding coat on ferrous metal surfaces which comprises the following sequence of steps: (l Wetting the metal surface with a solution containing a dichromate selected from the class Which consists of barium, calcium, magnesium and zinc dichromates, the solution also containing a reducing agent for said dichromate, said reducing agent being present in amount sufficient to reduce substantially completely the hexavalent chromium present, the solution also containing a phosphoric acid compound selected from the class which consists of barium, calcium, magnesium and zinc dihy-drogen 4. As a new product, a ferrous metal base carrying on the surface an adherent, micro'crystalline chromitephosphate bonding coat containing up to 49 percent of divalent metal phosphates and 51 to- 98 percent. of divalent metal 'ch-romites, the divalent metal ions of the coating being selected from the class which consists of barium, calcium, magnesium and Zinc ions, the coating being substantially free from hexavalent chromium, the

coating being capable of withstanding deep-drawing and repeated flexing through 360. n

5. Process for producing an adherent, micro-crystalline chromite-phosphate bonding coat on a ferrous metal base which comprises the following sequence of steps: 1) wetting the metal surface with a solution containing a dichromate selected from the class consisting of barium,

calcium, magnesium and Zinc dichromates, the solution also containing an organic reducing agent for said dichromate, said reducing agent being present in amount sufficient to reduce substantially completely the hexavalent chromium present, the solution also containing a phosphoric acid compound selected from the class consisting of barium, calcium, magnesium and Zinc di-hydrogen phosphates and free phosphoric acid, the phosphate being less in amount than the dichromate in the solution, and (2) destroying substantially completely hexavalent chromium compounds and reducing agents present in the film of said dichromate solution adhering to the metal surface by heating to a temperature above 100 C., the

residue remaining on the metal surface constituting an adherent, micro-crystalline chromite-phosphate bonding coat.

6. Process for producing an adherent, micro-crystalline chromite-phosphate bonding coat on a ferrous metal base which comprises the following sequence of steps: 1) Wetting the metal surface with a solution containing a dichromate selected from the class consisting of barium,

calcium, magnesium and Zinc dichromates, the solution 7 also containing a reducing agent for said dichromate which is a hypophosphorous acid compound, said-reducphosphates and free phosphoric acid,'the phosphate being less in amount that the dichromate in the solution, and (2) destroying substantially completely hexavalent chromium compounds and reducing agents present in the film of said dichromate solution adhering to the metal surface by heating to a temperature above 100 C., the residue remaining on the metal surface constituting an adherent, micro-crystalline, chromite-phosphate bonding coat.

2. The process of claim 5 wherein the reducing agent comprises a polyhydroxy alcohol.

3. Process for producing an adherent, micro-crystalline chromite-phosphate bonding coat on ferrous metal surfaces which comprises the following sequence of steps:

(1) freeing the metal surface from waterbreaks by pre-' heating the metal to a temperature substantially above 100 C., (2) wetting the surface of the hot metal with a solution containing a dichromate selected from the class which consists of barium, calcium, magnesium and zinc dichromates, the solution also containing a reducing agent for said dichromate, said reducing agent being.

present in amount sufficient to reduce substantially completely the hexavalent chromium present, the solution also containing a phosphoric acid compound selected from the 7 class whichconsists of barium, calcium, magnesium and zinc dihydrogen phosphates and free phosphoric acid, the phosphate being less in amount than the dichromate in the solution and, (3) destroying substantially completely hexavalent chromium compounds and reducing agents present in the film of dichromate solution adhering to the metal surface by heating to a temperature above 100 C., the'necessary heat and temperature being supplied by the preheated metal, the residue remaining 'on the surfaceof the metalf constituting an adherent, microcrystalline chromite-phosphate bonding coat.

ing agent being present in amount sufficient toreduce substantially completely the hexavalent chromium present, the solution also containing a phosphoric acid compound selected from the class which consists of zinc di-hydrogen phosphate and free phosphoric acid, the phosphate being less in amount than the dichromate in the solution, and

(2) destroying substantially completely hexavalent chromium compounds and reducing agents present in the film of said dichromate solution adhering to the metal surface by heating to a temperature above C., the residue remaining on the metal surface constituting an adherent, micro-crystalline chromite-phosphate bonding coat.

7. Process for producing an adherent, micro-crystalline water-repellant chromite-phosphate bonding coat on ferrous metal surfaces which comprises the following sequence of steps: (1) wetting the metal surface with a,

solution containing a dichromate selected from the class which consists of barium, calcium, magnesium and zinc 'dichromates, the dichromate solution also containing a reducing agent for said dichromate, said reducing agent.

being present in amount sufficient to reduce substantially completely the hexavalent chromium present, said reducing agent including up to 0.20 percent, based on the weight of the solution, of a phosphoric acid mono-ester of astraight-chain alcoholwontaining from about 12 to about 18' carbon atoms, said dichromate solution also containing a phosphoric acid compound selected from the'class which consists of barium, calcium, magnesmrn and zinc'di-hyclrogen, phosphates, thephosphate being" less in amount than the dichromate in the solution, and (2) destroying substantially completely hexavalent chromium'compounds and reducing compounds present in the film of dichromate solution adhering to the metal surface by heating to a temperature above 100 C., the residue remaining on the surface constituting an adherent, micro crystalline water-repellant chr0mite-phosphate bonding coat.

8. The product of claim 4 wherein the chromite-phosphate bonding coat contains a substantial amount of water-repellant, insoluble organic substances produced by the oxidation by dichromate of a phosphoric monoester of a C to C open-chain alcohol.

References Cited in the file of this patent UNITED STATES PATENTS Curtin Aug. 6, 1940 Roesnet et al. Oct. 13, 1942 Thomas et a1 Jan. 29, 1946 Tanner Dec. 10, 1946 Schuster et a1 Oct. 23, 1956 

1. PROCESS FOR PRODUCING AN ADHERENT MICRO-CRYSTALLINE CHROMINE-PHOSPHATE BONDING COAT ON FERROUS METAL SURFACES WHICH COMPRISES THE FOLLOWING SEQUENCE OF STEPS (1) WETTING THE METAL SURFACE WITH A SOLUTION CONTAINING A DICHROMATE SELECTED FROM THE CLASS WHICH CONSISTS OF BARIUM, CALCIUM, MAGNESIUM AND ZINC DICHROMATES, THE SOLUTION ALSO CONTAINING A REDUCING AGENT FOR SAID DICIROMATE, SAID REDUCING AGENT BEING PRESENT IN AMOUNT SUFFICIENT TO REDUCE SUBSTANTIALLY COMPLETELY THE HEXAVALENT CHROMIUM PRESENT, THE SOLUTION ALSO CONTAINING A PHOSPHORIC ACID COMPOUND SELECTED FROM THE CLASS WHICH CONSISTS OF BARIUM, CALCIUM, MAGNESIUM AND ZINC DIIHYDROGEN PHOSPHATES AND FREE PHOSPHORIC ACID, THE PHOSPHATE BEING LESS IN AMOUNT THAT THE DICHROMATE IN THE SOLUTION AND (2) DESTROYING SUBSTANTIALLY COMPLETELY HEXAVALENT CHROMIUM COMPOUNDS AND REDUCING AGENTS PRESENT IN THE FILM OF SAID DICHROMATE SOLUTION ADHERING TO THE METAL SURFACE BY HEATING TO A TEMPERATURE ABOVE 100*C. THE RESIDUE REMAINING ON THE METAL SURFACE CONSTITUTING AN ADHERENT, MICRO-CRYSTALLINE, CHROMINE-PHOSPHATE BENDING COAT. 